This invention relates to a process and composite molding materials for the manufacture of products from reinforced polyester resins and, more particularly, polyester resin systems which are reinforced with glass fibers. In accordance with the process, encapsulated reaction additives such as initiators remain functionally isolated in the blended molding materials until released by preselected process conditions. Products manufactured from reinforced polyester resins are widely used in automotive, appliance, and other industries.
In the processes now employed, polyester resin reactants are added to a large mixer along with the additives, such as promoters or accelerators, inhibitors, pigments, stearates, fillers, thermoplastic profiling compounds, and the initiator or catalyst. The material is intensively mixed with high energy shearing for perhaps 20 minutes and then it is made into sheet molding compound (SMC), thick molding compound (TMC) or bulk molding compound (BMC). For convenience, reference to SMC also contemplates TMC unless otherwise indicated.
With SMC, the polyester resin reactant, cross-linking monomers and fillers are first intensively mixed and then the mixture is disposed over a mat of reinforcing glass fibers which in turn rests on a sheet of protective polyethylene. A second sheet of protective polyethylene is placed on top of the fiber/resin mixture to make a sandwich which can be rolled as noted below. Optionally, continuous lengths of glass fiber roving or mat may be disposed between sheets of compound, which in turn are covered, top and bottom, with protective polyethylene film. The film-protected SMC composite is passed between rollers which knead the composite in order to thoroughly mix and wet the glass fiber with resin reactants. The SMC is up to perhaps 1/4 inch thick. There are special high strength forms of SMC based upon particular fiberglass reinforcement characteristics. The TMC comes in sheet form like SMC, but may be up to several inches thick. A thickener such as magnesium oxide is included in the mixture so that, after the mixture has been mixed into the glass fiber mat, the viscosity will increase as the SMC is aged.
With BMC, short glass fibers 1/8 to 11/4 inches long are the reinforcement and are added at the time of mixing of the polyester resin reactant, cross-linking monomer and fillers. This is an intensive mixing process. BMC has a consistency similar to that of modeling clay and is extruded into logs or ropes, or pelletized, or may be used right out of the mixer. After mixing, BMC composite material may be wrapped with polyethylene film or placed in a tube enclosure for convenience of handling.
The ready-to-mold resin and reinforcement composites in the form of SMC and BMC materials are made up intermittently in large batches in accordance with production demand. After mixing, they are stored or matured under controlled conditions until they are used. Because the initiator is mixed into the compound, the compounds are partially cured and gelled, which increases their viscosity, and they continue to cure slowly in storage. After one or two months in storage, they become too cured or viscous to use and must be discarded. They thus have limited shelf life, depending upon storage conditions.
The composite materials may be prepared for use at a molding site which is remote from the manufacturer's mixing facility, or they may be prepared for sale to third-party molders who do not mix their own composite materials. In either case, the period and conditions of storage following mixing for off-site molding tend to be less controlled than those for on-site molding. The viscosity and degree of cross-linking of such composite materials shipped into the site will tend to correspondingly vary from the desired values and characteristics. These problems are particularly troublesome when the storage period of the composite material is intended to complement the mixing process by providing a predetermined further viscosity increase and/or degree of cross-linking. Accordingly, irregularities in the storage and/or subsequent handling of the composite materials may result in variations greater than those normally associated with mere quality control and such variations may comprise deficient as well as excessive viscosity and cross-linking characteristics.
A further problem occurring during the storage and handling of the composite materials is the formation of localized sites of excessive cross-linking in the composite materials. These are believed to occur at locations where the materials are exposed to relatively high pressures, such as at the support points resulting from the stacking of the materials. The localized sites of cross-linking are associated with defects in the molded products.
The foregoing storage and control problems are compounded by the lack of uniformity of the composite product resulting from the mixing process itself, particularly when the composite material is formulated to cure rapidly during molding in order to minimize the molding cycle or time. With respect to the latter, formulations have necessarily been tempered heretofore by the risk of premature cure during storage. These matters are discussed below.
In the course of the mixing, the temperature increases because of the energy put into the compound and because the reaction is exothermic. The intensity and length of the mixing process must be restricted to avoid excessive premature cure of the compound. Water jacket cooling techniques may be employed, but the mixing operation remains an art with variation of resin materials and initiators, as well as the possible use of promoters or accelerators and inhibitors. Often, the mixing process is simply terminated just prior to a critical temperature (e.g., 32.degree. C.).
One problem with SMC and BMC materials is that because of the restricted conditions under which they are mixed, it often happens that the initiator and fillers are not completely distributed throughout the mixture. This frequently occurs if the temperature of the mixture increases too much and the mixing operation has to be cut short before the additives are completely mixed in.
SMC is, for the most part, molded in matched metal die compression molds. It usually is about 24 inches wide and is weighed and cut into suitable lengths for insertion into the molds. BMC is likewise molded in compression molds. Pieces of BMC are cut off by weight and placed into molds. Charges can weigh as much as 30 pounds or more. BMC can be preheated in a screw and injected into the mold. It can also be injection-molded with a plunger. SMC, TMC, and BMC materials can also be molded in transfer molds. At this writing, most production uses either SMC or BMC compounds. The use of TMC is limited.
It is desirable for cost purposes to minimize the molding cycle or time, which tends to increase with the weight of the charge to the mold. To that end, increased amounts of initiators are used in combination with inhibitors, which act as free radical traps and tend to prevent premature initiation of polymerization. Preferably, the effect of the initiator is depressed during the storage of the compounds to improve shelf life, as well as during the mold filling process. However, at the desired point of cure, the initiators should cause rapid cure at high temperatures. Heretofore, these ideal conditions have been sought through the use of combinations of initiators and inhibitors, as well as promoters or accelerators, which tend to lower the decomposition temperature of the initiator. Combinations of these reaction additives involve trade-offs in the physical properties of the cured resin. Further reference is made to U.S. Pat. No. 2,632,751, columns 1 and 2.
For the products molded from SMC and BMC materials, there have always been problems in filling the molds completely and in obtaining suitable surface finish of the molded parts, even though inhibitors are used to delay the curing reaction and viscosity increases. From 10-20% of the products so molded have to be hand-finished, which is expensive and time-consuming. Even with hand-finishing, the scrap rate for these molded products may be in the order of 50%. The molded products are usually painted, and for that they need what is called a Class A surface. Tiny defects show up when the surfaces of molded products are painted. Additionally, the products have to be washed and cleaned before they can be painted. In both the cleaning and painting steps, the products are heated back up to temperatures which approach those at which they were molded. Most products are between 85% and 90% cured when they come out of the compression mold. The heating for the washing and painting increases the degree of cure, but it also relieves stresses in the parts, causing warpage and distortion.
Curable resin compositions containing encapsulated catalysts are known. U.S. Pat. No. 3,860,565 teaches the encapsulation of the catalysts for isocyanate resins and identifies a number of other patents relating to curable resin systems with encapsulated catalysts. U.S. Pat. Nos. 4,101,501 and 4,362,566 to Hinterwaldner disclose the microencapsulation of a large number of compounds, including peroxide initiators, combined with microhollow spheres for filler compositions or dowel cement compositions. The encapsulated material is supposed to be released by pressure and the microhollow spheres are supposed to assist in the release. Some examples even shown an encapsulated peroxide initiator in a polyester resin system which are said to be unsatisfactory unless combined with the microhollow spheres.
Japanese Patent Publication SHO 57(1982)12017 discloses the encapsulation of peroxide initiators and/or accelerators in a polyester resin system for press molding in which low temperatures and pressures are involved. While a number of encapsulating materials are suggested, paraffin wax is used in the examples.
None of these references teach the encapsulation of a peroxide initiator in a polyester resin system wherein the initiator is released by reason of internal vapor pressure developed by a liquid phase within the capsule at elevated temperatures and at conventional molding pressures. These initiators are usually organic peroxides in the form of volatile liquids. Such initiators are toxic and difficult to handle.
The processes for the micro-encapsulation of materials are well known, and have been well known ever since the end of World War II. Reference is made to "Microcapsule Processing and Technology," Asaji Kondo, edited and revised by J. Wade Van Valkenburg, Marcel Decker, Inc., New York, N.Y. (1979), and "Capsule Technology and Microencapsulation," Noyes Data Corp., Park Ridge, N.J. (1972).